This invention relates to a hardened calcium silicate material which is lightweight and has a high strength and a high durability and a process for producing the same.
With the recent demand for lightweight buildings, there have been required incombustible and lightweight building materials. As such building materials, autoclaved lightweight concretes (ALCs) and fiber-reinforced calcium silicate board have been generally employed. Autoclaved lightweight concretes are produced by using cement and powdery silica as the main materials optionally together with quick lime powder, gypsum, etc., adding water thereto to give a slurry, molding the slurry in a formwork, and then curing it in an autoclave. Because of being lightweight (i.e., having a specific gravity of about 0.5 to 0.6) and containing a large amount of tobermorite (5CaO.6SiO2.5H2O) having a high crystallinity, these autoclaved lightweight concretes are excellent in long-term weatherability. Thus, they are widely employed as exterior wall materials, floor materials and interior wall materials in buildings.
The compressive strength of these autoclaved lightweight concretes ranges from 40 to 50 kgf/cm2. On the other hand, these materials have a low flexural strength, which is an important physical factor as a panel, of about 10 kgf/cm2. Thus, steel bars are provided within autoclaved lightweight concretes for reinforcement to thereby secure the design strength. The need of providing the steel bars restricts the production of autoclaved lightweight concretes in several points. That is to say, such an material should have a thickness of at least 50 mm, which brings about an increase in the weight per unit area in practice. There arises another problem that materials in a complicated shape can be hardly manufactured. Moreover, autoclaved lightweight concretes contain a large number of bubbles of about 1 mm in diameter, which cause another serious problem of being brittle and poor in processability (for example, surface smoothness and sawing properties).
Therefore, attempts have been made to improve the strength of autoclaved lightweight concretes by, for example, regulating the bubble size distribution, elevating the ratio of closed cells, or elevating the tobermorite crystallinity. However, none of these attempts can establish sufficient results so far. Further, JP-A-7-101787 discloses a technique relating to ALC which is made lightweight without resort to bubbles, and reports a building material having a compressive strength exceeding 200 kgf/cm2 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) However, this method can give a specific gravity of 0.7 at the lowest, which is still insufficient as a lightweight building material.
On the other hand, fiber-reinforced calcium silicate board is produced by reacting crystalline or amorphous silicate with lime and then hardening together with reinforcing fiber in an autoclave. Uses of the fiber-reinforced calcium silicate board are roughly classified into heat insulating materials having a specific gravity of 0.3 or less, flame-resistant coating materials having a specific gravity of 0.3 to 0.4 and flame-resistant building materials having a specific gravity of 0.6 to 1.2. For molding, the filter press method is used in case where specific gravity is 0.4 or less, while the fabrication method is used in case where specific gravity is 0.6 or more. Hardening reaction is carried out with the use of an autoclave in each case. The hardened materials contain, as the main components, tobermorite, xonotlite, low-crystalline calcium silicate hydrate (tobermorite gel or CSH gel, hereinafter referred to simply as CSH) and the like, in addition to the fiber.
Because of containing a large amount of fiber (5 to 20% by weight) , this fiber-reinforced calcium silicate boards are excellent in flexural strength and toughness and show a high processability. However, the boards have a high water absorptivity and a high drying shrinkage ratio, which makes them poor in dimensional accuracy. The boards have additional problems; for example, there are a lot of dust falling off from the boards and, frequently there are flaws on the board, which are caused by the insufficient hardness of the surface of the boards. Moreover, fiber-reinforced calcium silicate board containing CSH as the main component is poor in weatherability and durability. Since use of the fiber-reinforced calcium silicate board as an exterior building material is restricted by these problems, they are mainly employed as building materials for interior use. For example, Japanese Patent No. 2514734 discloses a technique relating to a molded calcium silicate material composed of tobermorite, CSH, quartz and reinforcing fiber and reports a building material having a specific gravity of 0.55 and a flexural strength of 100 kgf/cm2 or more. In this method, a silicate material and a lime material are brought into contact at a temperature of 50xc2x0 C. or below to thereby elevate the tobermorite content in the molded material. However, the tobermorite in the molded calcium silicate material produced by this method has a very low crystallinity, compared with tobermorite generally observed in autoclaved lightweight concretes. Thus, the molded calcium silicate material has only an insufficient weatherability, in particular, resistance against neutralization due to carbon dioxide in the atmosphere, which makes it unusable as an exterior building material.
An object of the invention is to provide a hardened calcium silicate material which is lightweight (i.e., having a specific gravity of 0.2 or more but less than 0.7) but has a strength adequate for building materials without resort to any reinforcements such as steel bars, a long-term durability owing to its high crystallinity and an excellent processability such as a high surface smoothness and excellent sawing properties.
The inventors paid attention to the CaO/SiO2 ratio at the early stage of the reaction of starting materials, conducted intensive studies, and thereby completed the invention.
Accordingly, the invention relates to the follows.
(1) A hardened calcium silicate material characterized by having Ib and Ia, wherein a ratio of Ib to Ia is greater than 3, (wherein Ib represents the diffraction peak intensity of the tobermorite (220) plane in powdery X-ray diffractometry; and Ia represents the minimum diffraction intensity in the angle region located between two diffraction peaks in the tobermorite (220) and (222) planes;), having a specific gravity of 0.2 or more but less than 0.7, and being substantially free from bubbles having a maximum diameter which is greater than 200 xcexcm.
(2) The hardened calcium silicate material as described in the above (1) characterized in that the specific surface area measured by the nitrogen adsorption method is 60 m2/g or less.
(3) The hardened calcium silicate material as described in the above (1) or (2) characterized in that, among tobermorite diffraction peaks observed in powdery X-ray diffractometry, the ratio of the diffraction peak intensity I(002) of the (002) plane to the diffraction peak intensity I(220) of the (220) plane, i.e., I(002)/I(220) is 0.22 or more.
(4) The hardened calcium silicate material as described in any of the above (1) to (3) characterized in that, among pores measured by mercury porosimetry, the ratio of pores having a pore size of 1.0 xcexcm or more is from 1% by volume to 15% by volume (inclusive).
(5) A process for producing the hardened calcium silicate material as described in any of the above (1) to (4) characterized in that it comprises mixing a primary material and a secondary material in a slurry state, preliminarily hardening the obtained slurry at 40xc2x0 C. or above, and then curing in an autoclave at 160xc2x0 C. or above. The primary material is obtained by mixing a silicate material, a lime material and water, as the main component, at 40xc2x0 C. or above in such a manner as to give a CaO/SiO2 molar ratio of from 1.2 to 2.5. The secondary material contains at least one of a silicate material and a lime material or at least one of a silicate material and a lime material together with water, as the main component, in such a manner as to give a CaO/SiO2 molar ratio after mixing of from 0.6 to 1.5, a weight ratio of water to the total solid weight of from 1.0 to 5.0, and an amorphous silicate material content in the silicate materials of less than 20% by weight.
The invention is based on the findings that a hardened calcium silicate material having a low specific gravity can be obtained without resort to any foaming agents by mixing under definite conditions and then reacting a primary material, which is obtained by blending and reacting a silicate material, a lime material and water, as the main component, to give a CaO/SiO2 ratio within a definite range, and a secondary material selected from the same material species; and that such a hardened calcium silicate material contains a large amount of highly crystalline tobermorite and has a microstructure different from the conventional ones.
The invention is also based on the findings that these hardened calcium silicate materials show a strength several times higher than conventional autoclaved lightweight concretes while sustaining the high durability thereof, a largely improved cutout resistance which is one of the shortages of the conventional autoclaved lightweight concretes, and a high processability; and that these hardened calcium silicate materials are superior also in compressive strength and durability to the conventional fiber-reinforced calcium silicate board.